The practical reality is that sharing photos of some group event with the other people at that event is easy in Facebook, and hard to imagine how to do that without some analogous service. I'm aware that many people use Facebook for other odd things but this was Facebook's original use case, and the literal source of its name, and it is still useful for that.

Even if you could come up with some alternative or even decentralised way of doing that, this doesn't change the fact that you're addressing the purpose of Facebook, rather than proving your claim that "it serves no purpose". At university (pre-Facebook) I had a friend who ran a Facebook-like server for sharing photos amongst our group of friends. That need was already there and strong enough he wrote code for it.